Enantiomerically Pure Fullerenes as a Means to Enhance the Performance of Perovskite Solar Cells

The rapidly advancing improvements in perovskite solar cells (PSCs) are driven, in part, by the inclusion of suitable electron transport layers (ETLs) in high performance devices. Fullerene derivatives are particularly useful ETLs in PSCs, but many of the utilized fullerenes are present as isomeric mixtures. The opportunities presented by single‐isomer, single‐enantiomer fullerenes in PSCs are poorly understood. Here, inverted PSCs are prepared using bis[60]phenyl‐C61‐butyric acid methyl ester derivative (anti)16,17‐bis[60]PCBM, comparing the performance of enantiomerically pure material to the corresponding racemate. The single enantiomer devices are found to have an improved performance, giving a power conversion efficiency (PCE) of 23.2%, compared to 20.1% PCE for the racemate. It is also shown that enantiomerically pure PSC modules can be prepared with a state‐of‐the‐art PCE of 20.1%. Such excellent performance for the single enantiomer devices is accompanied by enhanced operational stability. This study thus provides strong evidence that single isomer ETLs can provide important improvements in PSC performance and it positions chiral fullerenes as an exciting material class moving forward.


Introduction
Perovskite solar cells (PSCs) are one of the most promising technologies for next generation photovoltaics, with rapid growth of power conversion efficiency (PCE) from 3.8% to the recently achieved 25.7% within 13 years. [1,2] Such rapid advancement has benefited from the development of improved materials, microstructures, interfaces, and device architectures. [3][4][5][6][7][8][9][10] provide excellent energy level matching to the perovskite active layer and electrodes, while having high electron affinity and high electron mobility. [16,[20][21][22] Beyond PSCs, fullerene derivatives are also widely used as electron acceptors in other organic optoelectronic devices. [23][24][25][26] Given the critical role of ETLs in PSC performance, tremendous efforts are ongoing for the discovery of new electron transport materials to achieve high PCE. [11,12,27,28] While these studies focus on the design of new molecular materials capable of acting as efficient ETLs, there are still important questions surrounding existing materials in use. Specifically, the addition of (often chiral) solubilizing groups to a range of ETLs can result in the generation of stereoisomeric mixtures. [17,[29][30][31][32][33][34][35][36][37] Individual components of such mixtures are rarely isolated or studied independently and therefore the impact of such heterogeneity on performance is mostly unknown. We have advocated the potential value of chirality in optoelectronic technologies, which includes the specific study of single enantiomer chiral organic semiconducting molecules. [38] C 70 -DPM-OE, bis C 70 -DPM-OE, bis C 60 -DPM-OE, C 60 -SAM, and bis[60]PCBM are all excellent chiral fullerene based ETLs layers for PSCs, which to date have solely been employed as racemates. [17,29,30] The importance of chirality on the performance of fullerene ETLs in PSCs is therefore unknown.
Given the potential importance of chiral composition on the performance of chiral fullerene derivatives, we report PSCs using single enantiomers (anti,S)16,17-bis[60]PCBM and (anti,R)16,17bis[60]PCBM (Figure 1), as well as the corresponding racemate, as the ETL in p-i-n structured devices. We find a higher electron extraction rate and improved electron mobility for enantiomerically pure ETLs, giving a promising PCE 23.2%, compared to 20.1% PCE for the racemate. To investigate future potential applications, enantiomerically pure PSC modules were also fabricated with an excellent PCE of 20.1%. Finally, we found enantiomerically pure ETLs to more effectively suppress ion migration and enhance the operational stability of PSCs. Taken together, these data advocate the further study of single enantiomer ETLs in high performance PSCs and related technologies.
We fabricated FAPbI 3 based inverted perovskite solar cells using single enantiomers (anti,S)16,17-bis[60]PCBM and (anti,R)16,17-bis[60]PCBM as the ETLs (Figure 2A). As shown in Figure S1 in the Supporting Information, the perovskite layer is a compact, uniform, and pinhole-free film. We also prepared comparable devices using the (anti)16,17-bis[60] PCBM racemate (i.e., a 50:50 mixture of R and S enantiomers). Despite these materials solely differing in chiral composition (i.e., with the same molecular structure), we found PSCs based on pure enantiomers and the racemate to perform differently. Since anti S and anti R devices show identical performance ( Figure S2, Supporting Information), as would be expected, we focused on the use of anti S as a representative enantiomerpure sample for comparison to the racemate. As shown in Figure 2B and Figures S2-S6 and Table S1 in the Supporting Information, the champion racemate PSC exhibited an opencircuit voltage (V OC ) of 1.11 V, a short-circuit current density (J SC ) of 24.2 mA cm −2 , a fill factor (FF) of 0.75, and a PCE of 20.1%. For enantiomer-pure devices, the champion cell delivered enhanced V OC of 1.15 V, J SC of 25.2 mA cm −2 and FF of 0.80, contributing to the higher PCE of 23.2%. The J SC value obtained from the J-V characteristics was also in good agreement with that obtained from the external quantum efficiency (EQE) spectrum ( Figure 2C). The stabilized power output was 19.6% and 22.7% for racemate and enantiomer-pure cells, respectively ( Figure 2D). As shown in Figures S7 and S8 in the Supporting Information, no distinguishable hysteresis was observed in enantiomer-pure cells compared with racemate counterparts under forward and reverse voltage sweeps, implying effective charge extraction. [42] To estimate the upscaling ability of single enantiomer devices, we fabricated large-area perovskite modules following a blade coating technique. [43] As shown in Figure 2E, the perovskite modules delivered a champion PCE of 20.1% with an aperture area of 36.4 cm 2 , a V OC of 9.0 V, a short circuit current of 110 mA, and a FF of 0.74 (a geometrical FF of 0.87). The stablized power output (SPO) of our module is 19.5% ( Figure 2F), which is comparable to the highest PCE reported to date for perovskite modules with a similar size. [43] To understand the performance enhancements of the enantiomerically pure device over the racemate, we employed time-resolved photoluminescence (TRPL) to study the chargecarrier dynamics in perovskite films coated with racemate and enantiomer-pure ETLs. Figure 3A shows the TRPL spectra of glass/perovskite (pristine) and glass/perovskite/racemate or enantiomer-pure bis-[60]PCBM. The PL of pristine sample on glass decays monoexponentially and can be described by Equation (1) [44] where τ is the decay time constant, A is its corresponding decay amplitude, and B is a constant. The corresponding decay time of the pristine perovskite is 132.4 ns. When we added the ETL on top of the pristine samples, the TRPL transients show a biexponential decay with a relatively fast initial decay followed by a subsequent slower decay ( Figure 3A). We fitted these TRPL curves with a biexponential decay function (Equation (2)) [22] exp e xp where τ 1 and τ 2 are the fast and slow decay time constants, respectively, A 1 and A 2 are their corresponding decay amplitudes, and B is a constant. The average lifetime (τ avg ) was estimated from the fitted curve data according to Equation (3) [45] avg 1 1 The fitted parameters are summarized in Table 1. The presence of fast decay in the PL transient spectra of pristine perovskite with ETL suggests efficient charge transfer (rather than detrimental recombination). [46][47][48] The enantiomer-pure sample exhibits much faster decay than the racemate sample, indicating the enhanced charge transfer. This set of analyses confirms enantiomer-pure ETL provides a means to more efficiently transport electrons generated in perovskite, accounting for the enhanced solar cell efficiency. [49][50][51][52][53]8] To understand this enhanced electron transport, the electron mobilities of the pure enantiomer and the racemate were determined via space charge limited current (SCLC) measurement and were calculated from Child's law in Figure 3B. The enantiomer-pure film has a higher electron mobility 3.1 × 10 −3 cm 2 V −1 s −1 , more than twofold higher than the racemate (1.2 × 10 −3 cm 2 V −1 s −1 ). Since the racemic and enantiomer-pure bis-[60]PCBM only differ by chiral composition and not molecular structure, this mobility difference must stem from the macroscopic properties of the resultant films (e.g., molecular packing). To verify this, grazingincidence wide-angle X-ray scattering (GIWAXS) was conducted using both enantiomerically pure and racemic solid-state films. As shown in Figure 3C. the weak diffused halo observed for the racemate film implies a deficiency in long range order. Interestingly, for enantiomer-pure films, the near-isotropic diffraction rings in the GIWAXS patterns of the racemate are replaced with discrete Bragg spots ( Figure 3D), indicating the development of long range out-of-plane ordering. These data clearly demonstrate packing differences between the enantiomerically pure and racemic films, with the former being more ordered. This ordered packing of the enantiomers likely contributes to the higher electron mobility, compared to the less ordered packing of the lower mobility racemate.
To investigate the effect of enantiomerically pure ETLs on device stability, we subjected our PSCs to a set of rigorous stability tests. First, we studied the environmental stability of our unencapsulated solar cells stored in 65% relative humidity (R.H.) air at room temperature. As shown in Figure 4A and Figure S9 in the Supporting Information, the enantiomer-pure cell retains over 96% of its initial PCE after 1500 h storage while the racemate one remains 91% of its initial PCE. This slightly improved stability of enantiomer-pure cells indicates enhanced resistance against moisture for the enantiomer-pure ETL compared to the racemate. We believe the closer molecular packing in enantiomer-pure films provides a denser network against moisture invasion, attributing to the enhanced device stability. Then, we evaluated the stability of our encapsulated devices subjected to industry-relevant damp-heat tests where the PSCs were stored in 85 °C and 85% R.H. air following International Electrotechnical Commission (IEC) 61215:2016 protocol. As shown in Figure 4B and Figure S10 in the Supporting Information, the racemic and enantiomerically pure PSCs present similar stability, both remaining about 85% of their initial PCE  www.advenergymat.de www.advancedsciencenews.com after 1200 h damp-heat tests, implying the enantiomer-pure ETL has no significant effect on the thermal decomposition of underlying perovskite films.
To understand the operational stability of our PSCs, we performed maximum power point tracking (MPPT) of encapsulated PSCs under simulated 1-sun illumination in 40 °C ambient air for about 1200 h ( Figure 4C and Figure S1, Supporting Information). The racemic solar cells lost more than 20% of their initial PCE after 1200 h, while the enantiomer-pure PSC exhibits much-improved operational stability, remaining over 93% of their initial PCE after 1200 h operation. Huang et al. reported that an excess of charge carriers reduces the activation energy for ion migration and facilitates the migration of cations or anions. [54] As discussed above, the enantiomerically pure ETL shows more efficient charge transport and should results in the PSCs with less ion migration. To prove this, we aged fullstack devices under continuous illumination before peeling off their top electrodes and characterizing the structure of the active layers with X-Ray photoelectron spectroscopy (XPS) surface analysis. We found the I 3d signal increased substantially in the XPS spectrum of the aged ETL surface in racemic solar cells ( Figure 4D). This study confirms the ETL film based on enantiomerically pure bis[60]PCBM suppresses ion migration more effectively, contributing to the observed enhanced operational stability. We also studied the thermal and the operational stability of unencapsulated devices (Figures S12 and S13, Supporting Information). Although the unencapsulated devices are less stable than encapsulated counterparts, the enantiomer-pure ETL based PSCs decayed slower than the racemic ETLs in both cases, implying enantiomer-pure ETL can also stabilize unencapsulated solar cells.

Conclusion
We successfully fabricated inverted perovskite solar cells using enantiomerically pure fullerene ETLs. The resultant PSCs have a significantly improved efficiency of 23.2% and stability over 1200 h operation when using enantiomerically pure fullerene ETLs, compared to the racemate-based devices. These data strongly suggest the use of single isomer (enantiomer) ETLs is an exciting direction for PSCs with enhanced performance. The fact that we achieve one of the highest PCE records (20.1%) from a large area PSC module using an enantiomerically pure ETL, further advocates the potential promise of our approach. Given that there are numerous chiral ETLs under investigation, but currently only studied as racemic mixtures, this study positions chirality as a fruitful new avenue for PSC optimization.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.